Projects that will be undertaken in the next year are: OBJECTIVES: 1. Activation of delayed implanting mouse blastocysts in vitro appears to involve a significant increase in the rate of embryonic RNA synthesis. It is not clear whether this involves primarily increases in rRNA or mRNA. This will be determined by isolating rRNA from the embryos on sucrose density gradients and mRNA with oligo(dT) cellulose. Changes in the rate of synthesis of each class of RNA will be determined as embryos become activated. 2) The number of cells in delayed implanting mouse blastocysts increases as they become activated. Since the cells are blocked in the G1 phase of the cell cycle, they must undergo a round of DNA synthesis before cell division can occur. To determine when DNA synthesis is initiated, delayed implanting mouse blastocysts will be recovered at various intervals during the process of activation, incubated in (3H) thymidine and prepared for autoradiography. 3) DNA polymerase activity increases as delayed implanting mouse embryos are activated in vitro. It will be determined whether DNA polymerase activity increases when embryos are 'activated' in vitro and if so, whether synthesis of mRNA is necessary. This will be done by exposing the embryos to alpha-amanitin at various times during the activation process.